It is desirable in injection molding to urge the molten thermoplastic resin material into contact with the mold surfaces by exerting pressure on the resin. This aids the external surface of the plastic material in assuming the precise shape dictated by the mold surface. The pressure also assists in the filling of the mold space with molten resin even if the space is elongated or narrow and is normally difficult to fill.
In gas assisted injection molding, the articles are produced by injecting molten resin into the mold cavity and thereafter injecting a quantity of pressurized gas into the resin to fill out the mold cavity and form a hollow portion in the resin. The gas is preferably an inert gas such, for example, as nitrogen. Pressure is maintained on the gas in the hollow gas space within the resin until the resin has sufficiently set, whereafter the pressurized gas is released from the molded part hollow space and the molded part is removed from the mold cavity.
The gas assisted procedure is advantageous since the molded part produced utilizes somewhat less plastic material and is lighter than if the part was solid plastic. More importantly, the plastic in the gas assisted procedure will not have a tendency to shrink away from the mold walls during cooling since the internal gas pressure will keep it pressed against the walls, thereby minimizing or eliminating surface blemishes such as sink marks. Further, the gas assisted procedure eliminates the need to utilize the screw ram of the injection molding machine to pack out the mold during the molding cycle, thereby minimizing or eliminating molded in stresses in the molded part.
Various methods are known to inject gas into the resin to form the hollow body plastic article including injecting the gas into the sprue, injecting the gas into the runner, or injecting the gas directly into the mold cavity itself. The gas flow passageways are typically formed as round holes or as annular passageways. One problem with the prior art gas injection systems has been that the holes or passageways become clogged with resin during the course of carrying out the total molding operation. This clogging requires costly down time while the blockage is cleared. Clogging generally can occur during the initial stage of resin injection when pressures are relatively high. Partial clogging of the gas passage reduces the flow of gas in and out of the resin to the point where the molding operation produces scrap parts.
Various attempts have been made to address this clogging or blockage problem and attendant down time. One attempt is shown in U.S. Pat. No. 4,555,225 wherein a reciprocating pin is utilized to resist entry of resin into the gas passageway. Similar solutions are found in U.S. Pat. Nos. 4,740,150, 4,943,407, 5,044,924, 5,198,177, 5,198,238, 5,232,711 and 5,282,730. In each case, however, the solutions are complex and costly to construct and maintain.
Further attempts to address this blockage problem are shown in U.S. Pat. No. 5,151,278, 5,256,047 and French Patent No. 2,652,538.
In U.S. Pat. No. 5,151,278, a pin is located in a bore with an annular clearance to form a gas passageway of sufficient size for flow of gas in and out of the resin and the system relies on reciprocating action between the pin and the adjacent bore to wipe away any resin clogging the annular clearance. However, many resins used in the gas assisted injection molding process readily flow into such clearances. It is also likely for the pin to be out of concentricity with the bore with the result that the clearance between the pin and bore is exaggerated in certain eccentric portions of the clearance.
French Patent No. 2,652,538 discloses a pin located in a bore with a close fitting annular clearance which resists entry of resin into the clearance. However, the close fitting clearance limits the flow of gas into and out of the resin. This is most noticeable as the gas pressure in the resin is vented to atmosphere. As the gas pressure is reduced, the rate of flow of the gas is also reduced and the small clearance between the pin and the bore further reduces the rate of flow which in turn increases the cycle time for the total molding operation.